In this study we investigate the vulnerability of road networks to interdictions. We consider that an intentional attacker wants to maximize the congestion level on the network by interdicting some links of the network. The behavior of the drivers traveling on the network is assumed to follow user equilibrium traffic assignment which is affected by the interdiction initiated by the attacker. In this problem, we propose a method, to reduce the damage to lowest level. An attack-to-protect (ATP for short) method is developed to solve for optimal interdiction strategies and to explore the effectiveness of protecting the identified vulnerable links of the network. Numerical experiments are conducted to examine factors that influence the effectiveness of this method. Specifically, we compare the efficiency of this method when applied on different Cartesian grids. Managerial insights into the vulnerability and protection of road networks are drawn from the analysis.